A concern in the design of modern high-speed integrated circuits (“ICs”) is the delay differential between data and associated clock signals as they propagate through the IC. This propagation delay differential is referred to as “skew.” Skew related issues are particularly acute at high-clock speeds. Timing skew arises when the data arrives at a particular point in the IC delayed or early relative to the time its associated clock signal arrives at the same point. If timing skew is sufficiently large, then invalid data may be sampled or latched causing an error. Skew is a major constraint in the timing margins of the high speed ICs.
FIG. 1 illustrates a conventional test circuit 100 for measuring skew between a data buffer 105 and a clock buffer 110. The propagation delay of data signal 115 through data buffer 105 is Td while the propagation delay of clock signal 120 through clock buffer 110 is Tc+Tskew, where Tskew represents the skew or propagation delay differential between data buffer 105 and clock buffer 110. To measure Tskew at the outputs of data buffer 105 and clock buffer 110, application of data signal 115 is delayed (also referred to as “schmoo'd”) relative to application of clock signal 120, until flip flop 125 fails to validly recognize data signal 115 and therefore invalid data is latched through to output 130. At the point of failure, the applied delay Tapp between clock signal 120 and data signal 115 is measured and Tskew is calculated using the following equations:Tapp−Te=Tskew+Tsetup, where Te represents the expected delay if zero skew is present and Tsetup represents the setup time of flip flop 125. Assuming Tskew is much greater than Tsetup, that is if the setup time of flip flop 125 is negligible compared to Tskew, then Tskew approximately equals Tapp−Te. A disadvantage of the above conventional approach is that it is increasingly inaccurate if Tskew and Tsetup are of the same order of magnitude. In high-speed ICs, such as 1 Gbps serial links, Tsetup may approach Tskew, and therefore the conventional technique is inaccurate.